Atrial fibrillation or flutter and stroke: a Danish population‐based study of the effectiveness of oral anticoagulation in clinical practice

OBJECTIVES: A pooled analysis of randomized trials has shown that oral anticoagulation therapy reduces the risk of ischaemic stroke with 68% in patients with atrial fibrillation. We examined the effectiveness of oral anticoagulation on risk of stroke of any nature (fatal and nonfatal ischaemic and/or haemorrhagic stroke) in patients with nonvalvular atrial fibrillation or flutter living in the County of North Jutland, Denmark. DESIGN: Cohort study. SUBJECTS AND METHODS: We used the Hospital Discharge Registry covering the county (490 000 inhabitants) from 1991 to 1998 to identify 2699 men and 2425 women with atrial fibrillation or flutter, aged 60-89 years. Data on prescriptions of anticoagulation were obtained from the National Health Service. We defined use of oral anticoagulation as date of prescription or reiteration plus 90 days. Patients were followed in the County Hospital Discharge Registry until a diagnosis of stroke (fatal and nonfatal ischaemic and/or haemorrhagic stroke), emigration, death or the end of 1998. We used Cox regression analyses to estimate the relative risk of stroke associated with use of oral anticoagulation compared with no use, adjusted for age, diabetes and underlying cardiovascular diseases. RESULTS: Eight hundred and thirty-eight of 2699 men (31%) and 552 of 2425 women (23%) with atrial fibrillation had one or more recorded prescriptions of oral anticoagulation. The incidence rates of stroke were 31 per 1000 person-years of follow-up in men, and 30 per 1000 person-years of follow-up in women. The adjusted relative risks of stroke during anticoagulation were 0.6 95% confidence interval (CI) 0.4-1.0 in men, and 1.0 (95% CI 0.7-1.6) in women compared with nonuse periods. CONCLUSIONS: The effectiveness of oral anticoagulation in clinical practice may be lesser than the efficacy of oral anticoagulation reported from randomized trials.